Worksheet 1: What You Need to Know About Motion along the x-axis

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Worksheet 1: What You Need to
Know About Motion along the x-axis
In discussing motion, there are three
closely related concepts that you need
to keep straight. These are:
1) Position: x(t) –determines where the
object is on the x-axis at a given time t
2)Velocity: v(t) = x’(t) – determines how
fast the position is changing at time t
as well as the direction of movement
3) Acceleration: a(t) = v’(t) = x”(t)
determines how fast the velocity is
changing at time t; the sign indicates if
velocity is increasing or decreasing
If x(t) represents the position of a
particle along the x-axis at any time t,
then the following statements are
true:
1) “Initially” means when time t = 0
2) “At the origin” means when position
x(t) = 0
3) “At rest” means velocity v(t) = 0
4) If the velocity of the particle is
positive, then the particle is moving
to the right.
5) If the velocity of the particle is
negative, then the particle is moving
to the left.
6) To find average velocity over a time
interval, divide the change in
position by the change in time.
7) Instantaneous velocity is the
velocity at a single moment (instant)
in time.
8) If the acceleration of the particle is
positive, then the velocity is
increasing.
9) If the acceleration of the particle is
negative, then the velocity is
decreasing.
10) In order for a particle to change
directions, the velocity must change
signs.
11) One way to determine the total
distance traveled over a time interval is
to find the sum of the absolute values
of the differences in position between
all resting points. Here’s an example: If
the position of a particle is given by:
1
x(t) = 3 t  t  3t  4 , find the distance traveled
on the interval 0  t  6 .
3
2
11) To find when the particle changed
directions, we must first find the
times at which object stood still
which is when the velocity = 0.
11) cont’d
V(t) = x’(t) = t2-2t-3.
t2-2t-3 = 0
(t-3)(t+1) =0
t = 3 or t = -1 (which is not in [0,6]),
so t = 3 is the only time the object
changes directions.
11) cont’d.
Now, let’s see where the object starts,
comes to rest and ends.
X(0) = 4 (where the object starts)
X(3) = -5(where the object comes to
rest )
X(6) = 22 (where the object ends)
11) cont’d.
So the object starts 4 units to the right,
moves to -5 which is 9 units to the
left, then ends at 22 which is 27 units
to the right. So the total distance
traveled is 9 + 27 = 36.
In-class Assignment 3-4A
• Pg. 129 #1-

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